Attempt to index local 'hit' (a nil value) ?

Make sure your code is not going to error!

At the moment, there are tons of things that could cause your script to error. Here's a list of things that could cause your script to error,

• If a player isn't the thing that touched the script.

This is most likely the problem

... Okay that's all I found.

To make sure the part that touched the object is a player and not a part, you can do multiple things. You could,

• Check if the part's parent has a Humanoid

• Search for the parts parent's name in Players

• Simply check if hit is nil

Hmmm.... There are definitely more, but that's normally what I use. I'll be using the third option.

To add a check to see if the part's parent is a player's character, here's what I would do,

function onTouched(hit)
    hit = game.Players:GetPlayerFromCharacter(hit.Parent)
	if hit then
	    if hit.PlayerGui:FindFirstChild("SampleGui") ~= nil then
	        print("User already has menu!")
	    else
	        print("User recieved menu!")
	        local gui = game.ServerStorage.UserInterface.SampleGui:clone()
	        gui.Parent = hit.PlayerGui
	        wait(2)
	    end
	end
end

script.Parent.Touched:connect(onTouched)

That makes sure the part is a child of a character. If the part's parent is the character of a player, the code will run. If not, the code will not run.

That should hopefully work, if I read your question right.

Good Luck